The present invention generally relates to biological medical instruments and, more particularly, to an arthroplasty joint assembly for medically and articulately interconnecting opposed end regions of adjacent end-to-end bones of a patient.
There have been a number of techniques historically utilized to fuse smaller joints in a living body such as finger joints and toe joints. The use of K wires arranged in a cross or parallel fashion complimented with dental wiring to augment the fixation has also been proposed. Moreover, the use of a cone and cup technique coupled with a K wire fixation technique has also been proposed. The use of small screws and plates to supplement fixation of bony surfaces as well as several novel screw systems such as marketed by Howmedica and described in U.S. Pat. No. 5,417,692 have also been proposed for fixation of bones for fusion.
With the development of arthritic conditions of the metacarpal phalangeal joints, such as commonly seen in patients with rheumatoid arthritis, as well as arthritic conditions of the proximal interphalangeal joint, such as commonly seen in patients with rheumatoid and osteoarthritis, a number of devices have been proposed to alieviate the severe pain, deformity, and disability arising out of the arthritic destruction of these joints. One proposal involves the use of a silicone implant arthroplasty as a spacer coupled with reconstruction and realignment of adjacent ligaments and tendons to effect a satisfactory joint replacement in metacarpal and proximal interphalangeal joints of the hand. Such devices have also been used in the metacarpal phalangeal joint of the foot as well as other interphalangeal joints to a lesser degree. Use of these devices, however, in distal interphalangeal joints is frequently associated with failure due to the excessive forces across these joints.
While these artificial joints act as spacers, they are fraught with a difficulty of long-term failure due to the resorption of adjacent bone and the lack of permanent fusion or fixation. Thus, these joints act merely as spacers, and do not provide a degree of intrinsic stability, nor do they physically bond or become attached to the adjacent bone. Instead, they are surrounded with a membrane representative of a giant cell reaction or foreign body reaction.
U.S. Pat. No. 5,108,443 to P. Branemark and issued Apr. 28, 1992, discloses a technique wherein a screw assembly is placed in a bone that appears to be fairly uniform in cross-section between opposite ends thereof. This technique fails to take into account the proximal widening of bones known in the metaphysis, and the substantial mismatch of size and canal diameters that can occur in both normal and especially arthritic bone. Similarly structured devices also fail to allow a variability of size mismatch wherein the proximal phalanx of a hand may be substantially asymmetrical in size with the standard sizes available that would correspond to a metacarpal phalangeal joint within the enormity of population ranges sized by these systems. This lack of variability can lead to difficulty with stability of a finger joint implant, wherein the metacarpal phalangeal joint side may be quite rigid, but the proximal phalangeal side alternatively may be loose. Attempts to place a large implant to accommodate the loose proximal interphalangeal side may be fraught with excessive resection of bone on the metacarpal phalangeal joint side in order to accommodate the large size implant to obtain stability.
Grommets have also been proposed for use to provide rotational control, but do not take into account the size mismatch that can occur. This lack of variability of implants can be found both for enlargement of either the distal or proximal bone abnormally compared to the corresponding opposable bony surface. This can especially occur in metacarpal phalangeal joints, but alternatively can occur in proximal and distal interphalangeal joints of the hands. This lack of size variability due to the lack of modularity is a substantial problem and can lead to late failure with rotary changes and provide a substantial loss of stability and function as well as a decreased cosmetic result.
Thus, there is a need and a desire for an improved joint assembly that can be used as a distal interphalangeal joint, a proximal interphalangeal joint, or as a metacarpal phalangeal joint as required. There is also a need and a desire for a joint assembly that addresses long-term fixation needs, the rigidity of fixation relative to the bones, and the ease of interchangability in the case of failure with a solidly anchored support.
In view of the above, and in accordance with the present invention, there is provided an arthroplasty joint assembly for medically and articulately interconnecting first and second opposed end regions of lengthwise adjacent first and second bones. The joint assembly of the present invention includes a first screw having external generally uniformly pitched threading extending lengthwise from a generally pointed end of the screw for permanently anchoring the first screw in the medullary cavity of the first bone such that a joint end of the first screw projects a predetermined endwise distance beyond and away from the end region of the first bone and extends toward the second bone. The joint assembly further includes a second screw having external generally uniformly pitched threading extending lengthwise from a generally pointed end of the second screw for permanently anchoring the second screw in the medullary cavity of the second bone such that a joint end of the second screw projects a predetermined endwise distance beyond and away from the end region of the second bone and extends toward the first bone. The respective joint ends of the first and second screws are fixedly interconnected to each other by a connector. The connector permits the first and second screws and the bones anchored thereto to articulately flex relative to one another while maintaining the first and second bones in fixedly joined relationship relative to each other.
In a preferred form of the invention, each screw of the joint assembly defines a coaxial bore that opens to a trailing end of the screw and to the end region of the bone affixed to the screw. The bore has internal uniformly pitched threading extending along at least the lengthwise portion thereof.
In one form of the invention, the connector comprises a flexible generally U-shaped member, having two generally parallel leg portions that are flexibly joined to each other by a bight portion of the member. Fasteners interconnect the leg portions of the member to the joint ends of the first and second screws. In a most preferred form of the invention, a forced distribution member cooperates with a headed portion of each fastener and the leg portion of the U-shaped connector so as to broadly distribute forces developed by the fastener connecting the connector to the screw. In another form of the invention, the joint end of the first and second screw has an eye portion defining a bore having a closed margin that is defined entirely by the joint end of the respective screw. The connector comprises a generally cylindrical pin that fits through each bore to articulately interconnect the first and second screws to each other. In this form of the invention, the joint end of each screw can be either integrally formed with the screw or as a separate element which is threadably received within the threaded bore of the screw. In this form of the invention, the eye portion of each fastener is preferably configured with stops such that when the joint ends of the first and second screws are interconnected to each other, the respective stops on the joint end portions of the screws cooperate relative to each other to limit the angular articulation of the first and second screws relative to each other.
As mentioned, artificial joints that rely solely on fasteners or screws to secure the implanted artificial joint assembly in place often fail because the screw tends to loosen over time. Another aspect of the present invention relates to the use of grommets in operable association with each bone screw or fastener of the artificial joint assembly. The grommets are designed and configured to promote boney ingrowth of the surrounding bone tissue. As will be appreciated by those skilled in the art, the screws of the joint assembly of the present invention are formed of a material that allows early fixation of bone to the screw. While the bone screw initially fastens the joint assembly in place, the grommets of the joint assembly enable bony ingrowth for long-term stability of the joint assembly. With resorption of bone that frequently occurs around a screw, the grommet can provide for long-term stability if resorption does occur around the screw.
An alternative form of the invention is disclosed for those situations where greater stability is required, such as the index metacarpal phalangeal joint and proximal interphalangeal joint where substantial and oppositely directed forces are applied as seen in pinching type activities. This alternative type of joint assembly implant would be especially helpful in a patient prone to failure secondary to rotational or angular deformities that could occur with a U-shaped joint.
Numerous other features and advantages of the present invention will become readily apparent from the following detailed description, the appended claims, and the accompanying drawings.